Spaces, new lines, tabs, semicolons

Tip: Recall "Rules of Automatic Semicolon Insertion" and "The five restricted productions"

Answer:

i       ++;    // is valid

i       
++;    // is incorrect 

Explanation: post-increment/decrement belongs to "The five restricted productions". The five restricted productions are return, throw, break, continue, and post-increment/decrement. New line is not allowed.

Read it for details:
wikipedia.org: Whitespace and semicolons
pdf document: ECMAScript® 2015 Language Specification